Continuous copolymerization of ethylene with other comonomers is well known in the art. Two types of reactors which are widely used for such polymerizations are tubular reactors and stirred tank reactors. These reactors are quite different in their dimensions, and hence in the environment and state of motion of the reacting mixture. The nature of the polymers made in these various reactors is described in U.S. Pat. No. 4,351,931 for ethylene-methacrylic acid copolymers, which is hereby incorporated by reference.
The high-pressure process used for making polyethylene generally operates at conditions of pressure and temperature at which the polymer is soluble in the supercritical ethylene phase in the polymerization reactor. This process can be used to make copolymers and terpolymers of ethylene with more reactive polar comonomers like acrylic and methacrylic acids, and their esters. These newer polymer compositions are less soluble than polyethylene in the supercritical monomer phase, which can cause the buildup of an insoluble polymer layer on the internal reactor surfaces, commonly referred to as reactor "fouling". Excessive fouling can lead to decreased mixing quality, reduced product quality, and process shutdowns.
Conventional approaches to reduce reactor fouling have focused on increasing the solubility of the polymer in the supercritical monomer phase by increasing reactor temperature and pressure, and by the addition of polar solvents like acetone and methanol to increase the solubility of the polar comonomers in the supercritical ethylene phase. There are, however, serious limitations to these approaches. Increasing pressure beyond the design limits of the original polyethylene reactors and monomer compressors requires their replacement with expensive new equipment designed for the higher pressure. Increasing temperature promotes unwanted side reactions that limit attainable molecular weight and reduce product quality, for example color and strength. Addition of solvents also limits molecular weight, and requires additional investment and cost for solvent storage, feed, removal and disposal.
Another problem encountered in trying to control polymer solubilty in supercritical fluids is the high sensitivity of the solvent to relatively small changes in temperature or pressure. Slight changes in temperature and/or pressure can lead, in some cases, to sudden separating out of a now-insoluble copolymer, thus leading to potentially rapid reactor fouling.
For the production of random uniform copolymers in a continuous stirred tank reactor, it is desirable to keep the polymerizing mixture in one phase. Increasing the temperature and/or pressure for any monomer mix will decrease the likelihood of separating the mix into two phases. However, when the copolymerization is carried out in internally stirred reactors, the highly polar comonomers result in build up of polymer deposits on the inner surfaces of the reactor far more readily than with non-polar monomers, and at temperatures and pressures at which little or no deposits would occur with non-polar monomers. These deposits may be related to localized phase separation in localized colder regions of the polymerization kettle, such as the monomer inlet region or stirrer seal region.
The formation of these deposits adversely affects the stable operation of the reactor. In addition, with the passage of time, the deposits thermally crosslink to form an intractable gel. As gel particles subsequently are released from the reactor surfaces by the combined action of the shear of the stirrer and the flow of monomers through the vessel, the presence of those crosslinked particles in the copolymer tends to compromise the quality of the copolymer produced.
Comonomers with reactivities substantially equivalent to ethylene, such as vinyl acetate, normally will not exhibit this effect. In these cases, the relative proportion of each monomer incorporated into the polymer is similar to that of the monomer mix. In a continuous reactor operating under steady conditions, this means the composition of the monomer mix will be similar to that of monomers polymerized into the polymer. The polarity of the polymer will tend to be similar to that of the monomer mix, which substantially reduces the likelihood of reactor fouling.
U.S. Pat. No. 4,351,931 discloses high molecular weight uniformly random copolymers of ethylene and acrylic or methacrylic acid containing up to 10% of the acid together with a process for preparing such copolymers.
U.S. Pat. No. 4,252,924 discloses similar copolymers of ethylene and acrylic or methacrylic acid containing up to 35% of the acid component together with a process for preparing these copolymers.
U.S. Pat. No. 5,028,674, hereby incorporated by reference, discloses an improved process for the continuous polymerization of ethylene with certain polar comonomers, including .alpha.,.beta.-unsaturated carboxylic acids, wherein the addition of methanol, reduces or eliminates reactor fouling.
Various methods have been proposed to solve the problem of reactor fouling, including periodic reactor scraping or the addition of cosolvent methanol as in U.S. Pat. No. 5,028,674. However, none of these methods provides a completely satisfactory method of reducing reactor fouling, especially in the cases where the comonomers have significantly different rates of reaction from that of ethylene.
There is a need for a process for the copolymerization of ethylene with highly reactive comonomers in which a single phase is maintained in the reactor and with decreased separation or deposition on the inner parts of reactor vessel.